Especially for heavy road vehicles, such as trucks and buses, so called S-cam drum brakes are often used. Brake shoes provided with brake linings may be pressed apart against a brake drum by means of a generally S-shaped cam rotatable with a shaft (called the S-cam shaft) extending out of the brake drum. A lever, called the brake lever, attached to the S-cam shaft is connected to a piston rod of a preferably pneumatic brake cylinder in the vehicle underframe. Thus, at the admission of air under pressure to the brake cylinder a brake force will be transmitted from the piston rod via the brake lever to the S-cam shaft and the S-cam, which will press apart the brake shoes and thus apply the brakes.
When wear of the brake linings occurs, a longer stroke of the brake cylinder piston rod will be necessary before the brake is applied. It has become common to provide the brake lever with internal means for automatically turning a worm screw in engagement with a worm wheel, which is attached to the S-cam shaft, and thus adjusting the angular position of the brake lever in dependence on the brake lining wear; a so equipped lever is in the art called an automatic brake lever.
Most of the designs for such automatic brake levers are based on the so called piston stroke principle, i.e. the adjustment depends entirely on the piston rod stroke or in other words the angular movement of the brake lever past a certain value corresponding to the normal clearance or slack between the brake linings and the brake drum in the rest position.
More recently it has been found that different reasons speak for a more advanced principle--the so called clearance sensing principle. In this case the adjusting mechanism is able to differentiate between the piston rod stroke depending on the wear of the brake linings and that depending on the often considerable elasticity in the different parts between the brake cylinder and the brake drum. This means that the automatic adjustment reduces the clearance to the normal and desired value when it has become excessive, due for example to wear of the brake linings, whereas the mechanism ignores the influence of the elasticity.
Different requirements are imposed on a product of this kind. Generally speaking an automatic brake lever is a safety device working under extremely hard conditions as regards loads and environment. Further, the available space for the brake lever is often very limited. The reliability must be high and the periods between normal services as long as possible. Last but not least the price must be competitive.
Still further it is desirable to obviate the necessity for mounting the brake lever in a certain predetermined position, which is inconvenient not only at the initial mounting but even more after later servicing. The automatic brake lever shall thus preferably have a so called floating reference point or fixpoint.
The control arrangement for the brake lever ending with the clutch wheel on the worm screw shall have certain properties. Preferably the mechanism shall be slow-acting, which means that only a fraction of the whole excessive slack is to be reduced at each brake application. The control arrangement must therefore have a certain reduction, which may be obtained by a gearing between the control ring with its control arm and the clutch ring, namely in this case--coaxial with each other--the pinion, the carrier wheel and the control screw engaging the clutch ring perpendicular thereto.
Certain automatic brake levers with such a supplementary gearing are earlier known, such as U.S. Pat. No. 3 526 303, U.S. Pat. No. 4 484 665 and EP-A-30 766, which is regarded as the closest prior art.
A problem with a conventional brake lever of the kind where the necessary reference point for the slack adjusting function of the lever is found in a fix point in the vehicle chassis, is that the original mounting of the lever has to be done with certain accuracy in order to obtain the later proper function.
It is instead desirable to find a solution where the position of the reference point--and accordingly the angular position of the control arm relative to the lever housing--is immaterial for the proper function. The desirable condition can be called a floating reference point.
It is also desirable--in a slow-acting, clearance-sensing adjuster with a floating reference point--to have the adjustment performed during the return stroke, when the different parts are less strained than during the brake application stroke.